In recent years, the search for offshore oil and gas reserves has been carried into water depths considerably deeper than those from which most offshore oil and gas production has been conducted to date. Producing oil and gas from these deep water regions presents a host of technical problems. One of the most challenging of these has been the development of deep water platforms from which drilling and production activities can be conducted. Most current drilling and production of offshore oil and gas is conducted from platforms consisting of a work deck supported above the ocean surface by a rigid concrete or tubular steel structure which is fixed to the ocean bottom. Such platforms are well suited for a water depths up to 250-350 meters. However, as water depths exceed this, it becomes increasingly difficult and expensive to produce a structure which will rigidly resist the wave, wind and current loadings imposed on it. It is generally considered economically impractical to drill and produce oil and gas reservoirs in water depths beyond about 400 meters using a rigid structure.
For deep water applications, a number of types of offshore structures have been proposed which avoid the strong depth sensitivities of conventional rigid offshore structures. One such alternate structure is the tension leg platform (TLP). The general configuration of a TLP is illustrated in FIG. 1 of the appended drawings. A TLP has a buoyant hull which supports a work deck from which drilling and producing activities are conducted. The hull is moored to a foundation on the ocean bottom by a set of elongate tethers which are secured to the buoyant hull under tension. The tensioned tethers maintain the hull at a significantly greater draft than it would assume if free floating. The balance of forces imposed by buoyancy and the tensioned tethers limits the degree to which the TLP undergoes motion in response to forces imposed by waves, ocean currents and wind. It has been suggested that TLP's could be employed in water depths up to 3000 meters, whereas the deepest present application of a conventional rigid offshore drilling and production structure is in a water depth of approximately 410 meters.
Though TLP's avoid many of the disadvantages faced by conventional rigid platforms in deep water, they do present their own special problems. One area of TLP design and operation that has proven especially troublesome concerns the system for installing and tensioning the tethers. In most TLP designs proposed to date the tethers are installed by lowering them to the ocean floor through the columns of the TLP hull itself. To permit this, the tethers are made up of threaded tubular segments which are secured together section by section as the tether is lowered. This arrangement presents a number of problems. The TLP hull must be provided with heavy hoisting equipment to support the great weight of the tether as it is lowered or raised. This decreases the payload capacity of the TLP. Additionally, the full length of the columns of the TLP hull must be reserved for the tethers. This space could otherwise be used for other purposes, such as housing drilling and production equipment. Further, through-column tether installation is very time consuming. This increases the vulnerability of the TLP to adverse weather during the installation process.
It would be desirable to develop a TLP which avoids the need for through column tether installation.